Structural health monitoring (SHM) is an emerging field in which smart materials interrogate structural components to predict failure, expedite needed repairs, and thus increase the useful life of those components. SHM is a method of determining the health of a structure from the readings of an array of permanently-attached sensors that are embedded into the structure and monitored over time.
SHM can be performed in basically two ways, passive and active. Passive SHM consists of monitoring a number of parameters (loading stress, environment action, performance indicators, acoustic emission from cracks, etc.) and inferring the state of structural health from a structural model. In contrast, active SHM performs proactive interrogation of the structure, detects damage, and determines the state of structural health from the evaluation of damage extent and intensity. Both approaches aim at performing a diagnosis of the structural safety and health, to be followed by a prognosis of the remaining life. Passive SHM uses passive sensors which only “listen” but do not interact with the structure. Therefore, they do not provide direct measurement of the damage presence and intensity. Active SHM uses active sensors that interact with the structure and thus determine the presence or absence of damage. The methods used for active SHM resemble those of nondestructive evaluation (NDE), e.g., ultrasonics, eddy currents, etc., but they are used with embedded sensors. Hence, the active SHM can be seen as a method of embedded NDE.
In the application of this technological approach, the use of piezoelectric materials to convert electrical signals into acoustic energy (and vice versa) has found many industrial applications for sensors. One widely used active SHM method employs piezoelectric wafer active sensors (PWAS), which send and receive Lamb waves and determine the presence of cracks, delaminations, disbonds, and corrosion. Due to its similarities to NDE ultrasonics, this approach is also known as embedded ultrasonics. PWAS use a capacitor approach to create the electric field needed for excitation. PWAS have been applied to substrates and demonstrate the ability to detect and locate cracking, corrosion, and disbonding through use of pitch-catch, pulse-echo, electro/mechanical impedance, and phased array technology.
The embedded portion of the PWAS consists of physically separated piezoelectric thin plates with electrodes on their top and bottom surfaces. For array technology, the sensors must be positioned and embedded accurately relative to all other sensors in the array because geometry and location relative to one another is critical for the accuracy of the algorithms. Currently, PWAS use the substrate as a common ground. The array is created by embedding individual PWAS into a 1-D pattern, typically eight in a row with some predetermined separation, requiring each PWAS to be bonded separately in order to achieve the highest level of relative location accuracy. This approach thus is highly time consuming and often inconsistent between applicators of different competency.
Unfortunately, 1-D arrays have the limitation of being able to only see in 180 degree increments. The images received by the Embedded Ultrasonic Radar (EUSR) for 0 to 180 degrees are superimposed with the images received from the area for 180 to 360 degrees. Thus, what is left is a 180 degree field of view. However, the EUSR cannot distinguish between what occurs behind the array from what occurs in front of it.
Improvements in array technology are moving away from 1-D arrays to more complicated 2-D arrays, increasing the number of sensors by a factor of eight. For large arrays (8*8=64 sensors), the placement of the individual sensors is extremely time consuming and inaccurate. For example, U.S. patent Ser. No. 12/101,447 filed on Apr. 11, 2008, which is incorporated by reference herein, discloses that a plurality of individual sensors can be arranged in a pattern to form a 2-D phased array where each sensor is meticulously positioned and a wire is connected to each sensor.
The present invention addresses the disadvantages of current constructions and methods and provides improved methods of structural health monitoring.